Ab initio study of nonadditivity effects: spin-spin coupling constants for tetrafluoroethene in ternary pi complexes.

C(2)F(4) coupling constants have been evaluated at EOM-CCSD/(qzp,qz2p) in binary complexes with electron donors X (X = HLi, Cl(-), CN(-)) and with the electron acceptor FH, and in ternary complexes FH:C(2)F(4):X in which X and FH are located on opposite faces of the C(2)F(4) pi cloud. The electron donors X and the electron acceptor FH have opposite effects on (1)J(C-C), (1)J(C-F), (2)J(C-F), and (3)J(F-F) in binary complexes. Effects of X and FH on a particular coupling constant in a ternary complex are additive if the change in the coupling constant in this complex relative to C(2)F(4) is within 1 Hz of the sum of the changes in the corresponding binary complexes. This is the case for (1)J(C-F). Both positive and negative nonadditivities are computed for the remaining coupling constants. Although the values of most coupling constants lie between the values for FH:C(2)F(4) and C(2)F(4):X, that is not the case for (2)J(C-F), and the effect of FH is enhanced by the presence of X. Moreover, values of (3)J(F-F) trans and cis for FH:C(2)F(4):X when X is Cl(-) or CN(-) bonded through C are within 1 Hz of the values for the corresponding binary complex C(2)F(4):X. Significant differences can be found between the relative contributions of the PSO, FC, and SD terms to total J and to the nonadditivities of J in ternary complexes FH:C(2)F(4):X.